1. Field of the Invention
The present invention relates to a novel sensitizing dye and a photoinitiation system using the sensitizing dye. More specifically, the present invention relates to a photosensitive composition containing a photoinitiation system having high sensitivity and excellent stability. The present invention also relates to a photopolymerizable composition excellent as a material for a lithographic printing plate precursor capable of producing a printing plate by scanning exposure based on digital signals.
2. Description of the Related Art
In the related art, a PS plate having a construction such that a lipophilic photosensitive resin layer is provided on a hydrophilic support has been widely used as a lithographic printing plate, and a desired printing plate has been generally obtained by a plate-making method where masking exposure (surface exposure) is performed via a lith film and then the non-image area is dissolved and removed.
In recent years, digitization techniques of electronically processing, storing and outputting image information by using a computer have been widespread and various corresponding new image output systems have been put into practical use. As a result, there arises a demand for a computer-to-plate (CTP) technique of directly producing a printing plate without using a lith film, by scanning high directivity light such as laser light based on digitized image information, and production of a printing plate precursor suited for such a technique is now an important technical subject.
As one of the methods for obtaining such a lithographic printing plate by scanning exposure, a construction such that a photopolymerization system composition having excellent sensitization speed is used for the ink-accepting photosensitive resin layer (hereinafter referred to as a “photosensitive layer”) provided on a hydrophilic support has been heretofore proposed and this is already available on the market. The printing plate precursor having such a construction is facilitated in the development processing and favored with good plate-making and printing performances of giving excellent resolution, inking property, press life and scumming resistance.
The above-described photopolymerizable composition basically comprises an ethylenic unsaturated compound, a photopolymerization initiation system and a binder resin, where the photoinitiation system absorbs light to produce an active radical and this radical induces the addition polymerization of the ethylenic unsaturated compound to cause insolubilization of the photosensitive layer, as a result, an image is formed.
Most of the related-art proposals regarding the photopolymerizable composition capable of being scan-exposed use a photoinitiation system having excellent photosensitivity and many compositions are described, for example, in Bruce M. Monroe et al., Chemical Review, Vol. 93, pp. 435–448 (1993) and R. S. Davidson, Journal of Photochemistry and Biology A: Chemistry, Vol. 73, pp. 81–96 (1993).
With respect to the related-art CTP systems where a photopolymerizable composition comprising such an initiation system is used and a long wavelength visible light source such as Ar laser (488 nm) or FD-YAG laser (532 nm) is used as the light source, writing at a higher speed is demanded for the purpose of improving productivity in the plate-making step, but this has not yet been achieved because of insufficient output of the light source and insufficient sensitivity of the photosensitive material.
On the other hand, a semiconductor laser which uses, for example, an InGaN-base material and can effect continuous oscillation in the region from 350 to 450 nm has been recently put into practical use. The scanning exposure system using such a short-wave light source is advantageous in that the semiconductor laser can be produced at a low cost in view of its structure and therefore, an economical system having sufficiently high output can be constructed. In addition, as compared with the related-art systems using an FD-YAG or Ar laser, a photosensitive material having photosensitivity in the short-wave region and capable of being worked under brighter safe light can be used.
However, a photoinitiation system having sufficiently high sensitivity for scanning exposure in the short-wavelength region from 350 to 450 nm is not yet known at present.
Furthermore, the technique for obtaining a photoinitiation system having high sensitivity is still keenly demanded in the imaging field (see, for example, JP-A-2000-258910 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”), J. P. Faussier, Photoinitiated Polymerization-Theory and Applications: Rapra Review, Vol. 9, Report, Rapra Technology (1998), and M. Tsunooka et al., Prog. Polym. Sci., 21, 1 (1996)). The photoinitiation system comprising a sensitizing dye and an activator can generate an acid or a base other than the above-described radical by selecting the activator and therefore, this system is used, for example, in the image formation such as stereolithography, holography and color hard copy, in the production field of electronic materials such as photoresist, and in uses as a photocurable resin material for ink, coating material, adhesive or the like. In these industrial fields, in order to cause decomposition of the activator with good efficiency, it is demanded to find out a sensitizing dye having excellent light absorption and sensitization ability.